The present invention discloses a structural steel inner support system in a foundation pit which ensures the support density of the structural steel inner support beam of the inner support system for foundation pits and improves structural stability and reliability of inner support of foundation pits. The system includes a purlin assembly, a plurality of vertical force-guiding connecting members including support members, upper force-guiding connecting beams and lower force-guiding connecting beams in the foundation pit; and a structural steel inner support beam and a prestress-pressing member in the foundation pit.